A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting element because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors have been researched and developed particularly extensively. As a result, blue-ray-emitting light-emitting diodes (LEDs), green-ray-emitting LEDs and semiconductor laser diodes made of gallium nitride-based semiconductors have already been used in actual products (see Patent Documents Nos. 1 and 2).
In the following description, gallium nitride based compound semiconductors will be referred to herein as “nitride-based semiconductors”. Nitride-based semiconductors include compound semiconductors, of which Ga is replaced either partially or entirely with at least one of aluminum (Al) and indium (In), and are represented by the compositional formula AlxGayInzN (where 0≦x, y, z≦1 and x+y+z=1).
By replacing Ga with Al or In, the band gap can be made either wider or narrower than that of GaN. As a result, not only short-wave light rays such as blue and green rays but also orange and red rays can be emitted as well. That is why by using a nitride-based semiconductor, a light-emitting element that emits a light ray, of which the wavelength is arbitrarily selected from the entire visible radiation range, is realizable theoretically speaking, and therefore, they hope apply such nitride-based semiconductor light-emitting elements to image display devices and illumination units.
A nitride-based semiconductor has a wurtzite crystal structure. FIG. 1 illustrates the planes of a wurtzite crystal structure which are indicated by four indices (i.e., hexagonal indices). According to the four index notation, a crystal plane or orientation is represented by four primitive vectors a1, a2, a3 and c. The primitive vector c runs in the [0001] direction, which is called a “c axis”. A plane that intersects with the c axis at right angles is called either a “c plane” or a “(0001) plane”. It should be noted that the “c axis” and the “c plane” are sometimes referred to as “C axis” and “C plane”. FIG. 2(a) illustrates the crystal structure of a nitride-based semiconductor by ball-stick model and FIG. 2(b) indicates the positions of Ga and N in a nitride-based semiconductor crystal on a plane that intersects with the c-axis at right angles.
In the related art, in fabricating a semiconductor element using nitride-based semiconductors, a c-plane substrate, i.e., a substrate of which the principal surface is a (0001) plane, is used as a substrate on which nitride-based semiconductor crystals will be grown. In that case, as can be seen from FIGS. 2(a) and 2(b), there are a layer in which only Ga atoms are arranged and a layer in which only N atoms are arranged in the c-axis direction. Due to such an arrangement of Ga and N atoms, spontaneous electrical polarization is produced in a nitride-based semiconductor. That is why the “c plane” is also called a “polar plane”.
As a result, a piezoelectric field is generated in the c-axis direction in the InGaN quantum well of the active layer of a nitride-based semiconductor light-emitting element. Then, some positional deviation occurs in the distributions of electrons and holes in the active layer. Consequently, due to the quantum confinement Stark effect of carriers, the internal quantum efficiency of the active layer decreases, thus increasing the threshold current in a semiconductor laser diode and increasing the power dissipation and decreasing the luminous efficacy in an LED. Meanwhile, as the density of injected carriers increases, the piezoelectric field is screened, thus varying the emission wavelength, too.
As the In mole fraction of the active layer is increased in order to emit light rays falling within long wavelength ranges such as green, orange and red rays, the intensity of the piezoelectric field further increases and the internal quantum efficiency decreases steeply. That is why in an LED that uses a c-plane active layer, the wavelength of a light ray that can be emitted from it is said to be approximately 550 nm at most.
Thus, to overcome such a problem, people proposed that a light-emitting element be fabricated using a substrate, of which the principal surface is an m plane that is a non-polar plane (which will be referred to herein as an “m plane GaN based substrate”). As shown in FIG. 1, the m plane of a wurtzite crystal structure is one of six equivalent planes that are parallel to the c-axis and that intersects with the c plane at right angles. For example, an m plane may be a (10-10) plane, which is shadowed in FIG. 1 and which intersects with the [10-10] direction at right angles. The other m planes that are equivalent to the (10-10) plane are (−1010), (1-100), (−1100), (01-10) and (0-110) planes. In this case, “-” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar”.
FIG. 2(c) shows the positions of Ga and N atoms in a nitride-based semiconductor crystal in a plane that intersects with the m plane at right angles. Ga atoms and N atoms are on the same atomic plane as shown in FIG. 2(c), and therefore, no electrical polarization will be produced perpendicularly to the m plane. That is why if a light-emitting element is fabricated using a semiconductor multilayer structure that has been formed on an m plane, no piezoelectric field will be produced in the active layer, thus overcoming the problem described above.
In addition, since the In mole fraction of the active layer can be increased significantly, LEDs and laser diodes which can emit not only a blue ray but also green, orange, red and other rays with longer wavelengths can be made using the same kind of materials.
Furthermore, as disclosed in Non-Patent Document No. 1, for example, an LED which uses an active layer that has been formed on an m plane will have its polarization property affected by the structure of its valence band. More specifically, the active layer formed on an m plane mainly emits a light ray, of which the electric field intensity is biased toward a direction that is parallel to the a-axis. In the present description, a light ray, of which the electric field intensity is biased toward a particular direction, will be referred to herein as a “polarized light ray”. For example, a biased light ray, of which the electric field intensity becomes outstandingly high in a direction parallel to the X-axis, will be referred to herein as a “light ray polarized in the X-axis direction” and a direction that is parallel to the X-axis will be referred to herein as “polarization direction”. Also, if when a polarized light ray is incident on an interface, the light ray transmitted through the interface is still a polarized light ray, of which the electric field intensity is still as biased as the incident polarized light ray, then the light ray is regarded herein as “maintaining its polarization property”.
An LED which uses an active layer that has been formed on an m plane (which will be referred to herein as an “m plane light-emitting element”) emits mainly a light ray polarized in the a-axis direction as described above but also emits light rays which are polarized in c- and m-axis directions. However, those light rays that are polarized in the c- and m-axis directions have lower intensities than the light ray polarized in the a-axis direction. That is why in this description, the following discussion will be focused on the light ray polarized in the a-axis direction.
An m plane light-emitting element has such a polarization property, and therefore, is expected to be used as a light-emitting element which can emit a polarized light ray. For example, a liquid crystal display device uses the polarization property of a liquid crystal material, and therefore, needs to use polarized light as its light source. However, as no appropriate light sources that can emit polarized light are available so far, a traditional liquid crystal display device uses a light source such as an LED or a cold cathode fluorescent lamp (CCFL) and has the emitted light passed through a polarizer to obtain polarized light. According to such a configuration, however, most of the light emitted from the light source is cut off by the polarizer, and the light cannot be used efficiently. That is why if an m plane light-emitting element is used as a light source for a liquid crystal display device, then the light can be used far more efficiently, and there is no need to cut down the power consumption of the liquid crystal display device significantly or to provide a polarizer. Consequently, the manufacturing cost can be reduced as well.